The present invention relates to a semiconductor memory device, and particularly to a memory with a plurality of data input/output terminals.
Semiconductor memories have been used extensively for digital data processing because of their large memory capacities and high speed operations. In such memories, a large number of memory cells are arranged in a matrix form of rows and columns, and usually word lines and bit lines are arrayed in rows and columns, respectively. Through a selection operation, one of the word lines is first selected so that the memory cells on the selected word lines are enabled. Thereafter, one or more cells among the enabled memory cells are selected by selectively connecting one or more digit lines to an input/output circuit or circuits. Presently, the memories with one-bit input/output structure, such as 16-K word x 1 bit, 64-K word x 1 bit, have been mainly used. However, according to the recent progress of relatively small or medium scaled data processing system such as personal computers and desk-top computers, the necessity of multi-bit output type memories having two or more input/output terminals has become remarkably increased. This is due to the fact that data of desired bit length can be obtained easily with reduced number of memories. In order to obtain a predetermined memory capacity, a plurality of multi-bit output type memories are used in parallel wherein a plurality of input/output terminals of the respective multi-bit output type memories are commonly connected to the same number of system buses in known wired-OR manner and read-write control terminals of all the memories are connected in common to a system read-write control line. In this system, however, operations with respect to the input/output terminals cannot be controlled independently. Namely, during a write cycle, write operations with respect to all the input/output terminals are simultaneously conducted even when write operations on all the input/output terminals are not necessary.
In case of image display memories (VIDEO RAM) for storing image data, which memories have become popular in video field, data processing on a bit and that on a word are simultaneously performed. When the multi-output type memory is used in such video system, data once generated from the memory are re-written thereto according to a read-modify-write cycle. However, the operation is limited to the read-modify-write cycle and unnecessary writing is also performed on one or more input/output terminals in this instance so that the power consumption is inevitably increased.